The present invention relates generally to methods and apparatus for measuring liquid or granular solids product levels in containers. In particular, the present invention relates to stabilization of oscillators in low power pulsed radar level transmitters.
Low power pulsed radar level transmitters typically include transmit and receive oscillators that are offset from one another by a frequency offset. The stability of the radar level transmitter output depends on stabilizing the frequency offset. In one stabilization scheme, one of the two oscillators has been used to control the frequency of the other oscillator in an effort to stabilize the offset frequency. In another stabilization scheme, the frequency offset is sensed and used to control the frequency of one of the oscillators in an effort to stabilize the offset frequency. High power stabilization techniques with complex circuitry or crystal ovens cannot be used without exceeding the power supply limitations of an energy limited 4-20 mA loop that powers the level transmitter. The stabilization achieved within the power limitation has been inadequate, and a method and apparatus are needed that can stabilize a frequency offset without using power or energization in excess of that available from a two wire, 4-20 mA loop that provides all of the power or energization to the radar level transmitter.
Disclosed is a level transmitter that includes a first pullable oscillator circuit that provides a transmit frequency output, and a second pullable oscillator circuit that provides a receive frequency output. The receive frequency output is offset from the transmit frequency output by a frequency offset.
A first pulse generator receives the transmit frequency output and provides microwave transmit pulses couplable to the product level. A second pulse generator receives the receive frequency output and provides microwave gating pulses.
A gate receives the microwave gating pulses and is coupled to the product level for receiving microwave transmit pulses reflected back from the product level that are delayed by a reflection delay time. The gate provides a gated delayed pulse output. A controller receives the gated delayed pulse output and provides a level output representative of the product level.
A reference crystal oscillator circuit provides a reference frequency output that couples to the first and second pullable oscillators. The transmit and receive frequency outputs are pulled as functions of the reference frequency output.